The function of TMEM258 (transmembrane protein 258, Ensembl gene identifier ENSG00000134825) is as follows. Subunit of the oligosaccharyl transferase (OST) complex that catalyzes the initial transfer of a defined glycan (Glc(3)Man(9)GlcNAc(2) in eukaryotes) from the lipid carrier dolichol- pyrophosphate to an asparagine residue within an Asn-X-Ser/Thr consensus motif in nascent polypeptide chains, the first step in protein N-glycosylation (PubMed:31831667). N-glycosylation occurs cotranslationally and the complex associates with the Sec61 complex at the channel-forming translocon complex that mediates protein translocation across the endoplasmic reticulum (ER). All subunits are required for a maximal enzyme activity (PubMed:26472760, PubMed:27974209). Involved in ER homeostasis in the colonic epithelium (By similarity). {ECO:0000250|UniProtKB:P61166, ECO:0000269|PubMed:26472760, ECO:0000269|PubMed:27974209, ECO:0000269|PubMed:31831667}.